Human health depends on the relationship of the host with its microbiota. Microbial communities display coordinated behavior, and it is apparent that an understanding of human health must include knowledge of human-associated microbial communities. A step toward this goal is defining the genetics of communication within host-associated microbial communities. Using a model gut community, this work will test the hypothesis that bacterial genes that are induced when bacteria are part of a community maintain community integrity. I will use a promoter trap to identify bacterial genes in Enterobacter, a community resident, that are induced by the host gut or gut community, and use genetic, biochemical, and ecological approaches to determine how the genes contribute to the robustness of the native gut community. The cabbage white butterfly and its associated gut microbial community provides an outstanding model system for these studies, Using this system and tools developed in this proposal, we will gain insight into how the emergent properties of microbial communities determine host health.